Three main approaches are being used in our research: (1)\organ culture systems have been established for the sequential study of differentiation of neural tumors by morphological, immunological, and immunoradiometric methods. The kinetics parameters (total cell-cycle times, cell-cycle phases, growth fractions, and potential doubling times) have been determined by autoradiography on the C6 cell line and on human glioblastomas after steady-state of the explants in vitro has been attained: the growth fractions derived in these in vitro systems approximate those that have been reported in human malignant gliomas in vivo; (2)\the biological in vivo and in vitro properties of the mouse transplantable teratoma as a model for neuroepithelial neoplastic differentiation have been established. The relevance of this model lies in the expression of the full range of divergent neuroepithelial differentiation and therefore in its applicability to the study of embryonal central nervous system tumors in man; (3)\studies are continuing in human and experimental neural tumors of differentiation markers such as GFA protein, neurofilament protein, neuron-specific enolase (NSE), and cell surface membrane antigens specific for neuroepithelial cells and in the determination and quantitation of biogenic amines by high sensitivity microspectrofluorometry. The demonstration of GFA protein by immunoperoxidase has proved to be an indispensable tool for a better understanding of difficult central nervous system tumors and has been fruitfully used both for the delineation of new tumor entities and for the successful resolution of a number of cytological problems. Immunohistochemical and immuoradiometric studies on NSE as a marker for central nervous system tumors have defined its limitations for the recognition and interpretation of neuronal differentiation. (M)